1. Field of the Invention
The present invention relates to a superconducting current lead for supplying a current to a superconducting apparatus such as a superconducting magnet, or the like, cooled to a very low temperature by a power supply disposed at the room temperature.
2. Description of the Related Art
Superconducting magnets are used for research of physical properties or in magnetic resonance apparatuses, etc. and, in the future, such superconducting magnets are to be applied to magnetic levitated trains, nuclear fusion magnetic containment apparatuses, etc. Each of these superconducting magnets placed at a very low temperature, however, has a problem that heat penetrates a very low temperature region when a power supply placed at the room temperature supplies a current to the superconducting magnet. For example, liquid helium is an expensive liquid the price of which is no less than 1000 yen per liter. Furthermore, the refrigerator input electric power required for the refrigerator to re-liquefy liquid helium vaporized by penetration of 1 W heat is about 400 W even in an ideal condition and may reach 1000 W in a practical condition. For this reason, if the quantity of penetration of heat through the current lead is large, not only the cost increases with the purchase of liquid helium, or the like, but also the size and capacity of the refrigerator for re-liquefying increase. There would be no sense in using a current lead for a superconducting magnet which is intended to be reduced both in size and in electric power.
Therefore, the development of a low heat penetration type current lead has become an important development theme. With the discovery of oxide superconducting materials, a current lead using an oxide superconducting material disposed on the low temperature side of the current lead to reduce the quantity of penetration of heat into a very low temperature portion has been developed. For example, in Japanese Patent Unexamined Publication No. Sho-64-76707, an intermediate portion of a current lead for supplying a current to a superconducting apparatus in liquid helium is used as a thermal anchor for cooling to the temperature of liquid nitrogen, so that a superconductor (for example, Y--Ba--Cu--O) having a critical temperature of not lower than the boiling point (78 K) of Liquid nitrogen is used as a material for a lead portion between the thermal anchor and the superconducting apparatus. Further, in Japanese Patent Unexamined Publication No. Hei-5-109530, a superconducting portion of a current lead is composed of different kinds, for example, three kinds of conductors, that is, composed of a low temperature portion, an intermediate temperature portion, and a high temperature portion, which are connected to one another in a lengthwise direction by connection members. These low, intermediate and high temperature portions are different from one another in shape, superconducting characteristic (critical current density (Jc), critical temperature, resistance value in larger current density than Jc, and magnetic field and temperature dependences thereof) and composing method. The publications shows examples of materials for these low, intermediate and high temperature portions as follows. A conductor in which a Bi oxide superconductor layer having a high critical current density in a magnetic field intensity at 4.2 K is provided through a thin layer of Ag on a low thermal conductivity insulating substrate as a reinforcing material is used as the low temperature portion conductor. A conductor in which tape-like wire materials each composed of a core of an oxide superconductor coated with a coating material are collectively laminated is used as the intermediate temperature portion conductor. A yttrium oxide superconductor having a high critical current density (Jc) in 77 K is used as the core. An alloy composed of Au, and a small quantity of Pd added to Au is used as the coating material in order to suppress heat penetration. A conductor in which tape-like wire materials each composed of a core coated with a coating material are laminated and collected in the same manner as in the intermediate temperature portion is used as the high temperature portion conductor, in which a Tl oxide superconductor high in critical temperature is used as the core, and Ag small in resistance is used as the coating material. As described above, a material small in thermal conductivity is used in the vicinity of a superconducting magnet to thereby reduce heat penetration due to conduction. Further, in Japanese Patent Unexamined Publication No. Hei-4-218215, silver sheath oxide superconductors and a pipe formed of fiber reinforced resin material or of silver, copper, aluminum, nickel, stainless steel, an alloy thereof, etc. as a support member combined with the oxide superconductors are provided. The oxide superconductors and the pipe are stuck to each other by an adhesive agent layer or wound with a Teflon tape, or the like, so as to be fixed, by which the oxide superconductors and the support member move integrally at the time of thermal expansion and at the time of thermal contraction to fulfill stable superconducting characteristic against stress. In this case, the oxide superconductors are disposed so as to be parallel or helical in a longitudinal direction of the current lead.
On the other hand, the high temperature side of the current lead is, in most cases, constituted by a copper lead.
The conventional current lead using oxide superconductors, however, has the following problem. If the capacity of the current lead increases, that is, if the current value Increases, the self magnetic field generated by the current lead per se is intensified. The superconducting material used is classified into a bismuth type material and an yttrium type material. In each material, the critical current value in a magnetic field decreases extremely. Accordingly, in a large-current-purpose current lead using a bulk-like superconducting material having a uniform critical current density in the material, the necessary sectional area of the superconducting material increases. There arises a defect in that not only the superconductor portion of the current lead increases in size and becomes complex but also the quantity of heat penetration increases. The problem in lowering of the critical current value due to the self magnetic field has been not noticed in the prior art.